With X-men, they ocassionally split up the group into different teams-this started during the early 90s, when the second X-men series was introduced when X-factor rejoined the team. However, with all the crossovers they kind of eventually ignored this. They've tried it a few times since-notably in 2001 when the team was split into three, and I think they've done it recently as well (It's hard to keep up with X-men!)